1. Field of the Invention
The present invention relates to a planar magnetic element such as a planar inductor or a planar transformer.
2. Description of the Related Art
In recent years, electronic equipment of various types have been miniaturized. Magnetic elements such as inductors and transformers, which are indispensable to the power-supply section of each electronic component, can neither be made smaller nor be integrated with the other circuit components, whereas the other circuit sections have successfully been made much smaller in the form of LSIs. Therefore the ratio of the volume of the power-supply section to that of the other sections, combined together, has increased inevitably.
To reduce the sizes of the magnetic elements, such as inductors and transformers, attempts at reduction have been made, and small planar inductors and planar transformers have been achieved. A conventional planar inductor comprises a spiral planar coil, two insulation layers sandwiching the coil, and two magnetic plates sandwiching the coil and insulation layers. A conventional planar transformer comprises two spiral planar coils, used as primary and secondary windings, respectively, two insulation layers sandwiching these coils, and two magnetic layers sandwiching the coils and insulation layers. The spiral planar coils incorporated in the inductor and the transformer can be of either of the two alternative types. The first type is formed of one spiral conductor. The second type comprised of an insulation layer and two spiral conductors mounted on the two major surfaces of the insulation layer, for generating magnetic fields which extend in the same direction.
These planar elements are disclosed in K. Yamasawa et al, High-Frequency of a Planar-Type Microtransformer and Its Application to Multilayered Switching Regulators, IEEE Trans. Mag., Vol. 26, No. 3, May 1990, pp. 1204-1209. As is described in this thesis, the planar elements have a large power loss. Similar planar magnetic elements are disclosed also in U.S. Pat. No. 4,803,609.
It has been proposed that the thin-film process, is employed in order to miniaturize these planar magnetic elements.
Planar inductors of the structure specified above need to have a sufficient quality coefficient Q in the frequency band for which they are used. Planar transformers of the structure described above must have a predetermined gain G which is greater than 1 for raising the input voltage or less than 1 for lowering the input voltage, and must also minimize voltage fluctuation.
The value Q of a planar inductor is: EQU Q=.omega.L/R
where R is the resistance of the coil, and L is the inductance of the inductor.
The voltage gain G of a planar transformer without load is: EQU G=k(L.sub.2 /L.sub.1).sup.1/2 {Q/(1+Q.sup.2).sup.1/2 }
where k is the coupling factor between the primary and secondary windings, L.sub.1 and L.sub.2 are the inductances of the primary and secondary windings, respectively, the quality coefficient Q is .omega. L.sub.1 /R.sub.1, and R.sub.1 is the resistance of the primary-winding coil. The gain G is virtually proportional to Q when Q&lt;&lt;1, and has a constant value k (L.sub.2 /L.sub.1).sup.1/2 when Q&gt;&gt;1.
To increase the quality coefficient Q of the inductor, and to increase the gain G of the transformer thereby to limit the voltage fluctuation, it is necessary to reduce the resistance of, and increase the inductance of, the coil, as much as possible. In the conventional planar magnetic elements made by means of the thin-film process, however, the coil conductors, which need to be formed in a plane, cannot have a large cross-sectional area. Therefore, these elements cannot help but have a very high resistance and an extremely small inductance. Consequently, the conventional planar inductor has an insufficient quality coefficient Q, and the conventional planar transformer has an insufficient gain G and a great voltage fluctuation. These drawbacks of the conventional planar magnetic elements have been a bar to the practical use of these elements.
Of planar coils which can be used in planar inductors, spiral coils are the most preferable due to their great inductance and their great quality coefficient Q. In fact, planar inductors, each having a spiral planar coil, have have been manufactured, one of which is schematically illustrated in FIG. 1. As FIG. 1 shows, the planar inductor comprises a spiral planar coil shaped like a square plate, two polyimide films sandwiching the coil, and two Co-base amorphous alloy ribbons sandwiching the coil and the polyimide films and prepared by cutting a Co-based amorphous alloy foil made by rapidly quenching cooling the melted alloy. This planar inductor is incorporated in an output choke coil for use in a 5 V-2 W DC-DC converter of step-down chopper-type, as is disclosed in N. Sahashi et al, Amorphous Planar Inductor for Small Power Supplies, the National Convention Record, the Institute of Electrical Engineers of Japan 1989, S. 18-5-3. As is evident from the graph of FIG. 2A, two currents flow through this choke coil. The first current is a DC current which corresponds to the load current. The second current is an AC current which has been generated by the operation of a semiconductor switch. As the DC current increases, the operating point of the soft magnetic core, shifts into the saturation region of the B-H curve. As a result, the magnetic permeability of the magnetic alloy lowers, whereby the inductance abruptly decreases as is illustrated in FIG. 2B. As is evident from FIG. 3, the AC current becomes too large at the time the inductance sharply decreases. This excessive AC current is a stress to the semiconductor switch, and may break down the switch in some cases.
It is desired that the choke coil have its electric characteristics, such as inductance, unchanged even if a superimposed DC current flows through it. FIG. 4 is a graph representing the typical superimposed DC current characteristic of the choke coil, which is the relationship between the inductance of an inductor and a superimposed DC current flowing through the inductor.
In the case of a planar inductor, the conductor coil is very close to the soft magnetic cores and, hence, generates an intense magnetic field even if the current flowing through it is rather small. Thus, the soft magnetic cores are likely to undergo magnetic saturation. It will be explained how such magnetic saturation occurs in, for example, a planar inductor which comprises an Al--Cu alloy spiral planar coil, two insulation layers sandwiching the coil, and two magnetic layers clamping the coil and the insulation layers together.
The planar coil of this planar inductor is made of an conductor having a width of 50 .mu.m and a thickness of 10 .mu.m. The coil has 20 turns, and the gap between any two adjacent turns is 10 .mu.m. Each insulation layer has a thickness of 1 .mu.m, and either magnetic layer has a thickness of 5 .mu.m. The planar coil has a saturated magnetic flux density B.sub.S of 15 kG and a magnetic permeability .mu..sub.s of 5000.
Assuming that the Al--Cu alloy conductor has a permissible current density of 5.times.10.sup.8 A/m.sup.2, the permissible current Imax is 250 mA. The present inventors tested the planar inductor in order to determine the relationship between the current flowing through the coil and the intensity of the magnetic field generated in the surface of either magnetic layer from the current. The results of the test revealed that both magnetic layers were magnetically saturated when a current of 48 mA or more flowed through the Al-Cu alloy coil. It follows that, if this planar inductor is used as a choke coil, the maximum DC superimposed current is limited to 48 mA. This value is no more than about one fifth of the permissible coil current Imax. Inevitably, the magnetic layers will be readily saturated magnetically.
The limited DC superimposed current is a drawback which is serious, not only in the planar inductor used as a choke coil, but also in a planar transformer. In a planar transformer incorporated in, for example, a DC-DC converter of forward type or fly-back type, a pulse voltage of one polarity is applied to the primary coil. The magnetic layers are thereby saturated magnetically, abruptly decreasing the inductance of the transformer.
Hence, attempts have been made to provide a planar inductor and a planar transformer, which are designed such that the influence of the saturation of the magnetic layers is reduced, thereby to increase the maximum DC superimposed current of the device comprising the planar or transformer and to make an effective use of the magnetic anisotropy of the magnetic layers.
Planar coils can be classified into various types such as zig-zag type, spiral type, zig-zag/spiral type, and so on, in accordance with their patterns. Of these types, the spiral type can be provided with the greatest inductance. Hence, a spiral planar coil can be smaller than any other type having the same inductance. To form the terminals of a spiral planar coil, however, it is necessary to connect two spiral coils positioned in different planes by means of a through-hole conductor, or to use conductors for leading the terminals outwards. Hence, the process of manufacturing a spiral planar coil is more complex than those of manufacturing the other types of planar coils.
For electronic circuit designers it is desirable that planar magnetic elements to be incorporated in an electronic circuit have so-called "trimming function" so that their characteristics may be adjusted to values suitable for the electronic circuit. A magnetic element having a trimming function has indeed been developed, which has a screw and in which, as the screw is rotated, its position with respect of the core of the coil, thereby to vary the inductance of the magnetic element continuously. However, most conventional planar magnetic elements have no trimming function, for the following reason.
As is known in the art, the characteristics of planar magnetic elements greatly depend on their structural parameters and the characteristics of the planar coils and magnetic layers. These factors determining the characteristics of the magnetic elements depend on the steps of manufacturing the elements. Since these steps can hardly be performed under the same conditions, the resultant elements differ very much in their characteristics. Naturally it is desired that the elements be provided with trimming function. However, they cannot have trimming function because of their specific structural restriction.
Transformer with large output power is disclosed in A.F. Goldberg et al., Issues Related to 1-10-MHz Transformer Design, IEEE Trans. Power Electronics, Vol. 4, No. 1, January 1989, pp. 113-123.
As has been pointed out, planar magnetic elements small enough to be integrated with other circuit elements have not been produced, making it practically impossible to manufacture sufficiently small integrated LC-circuit sections, a typical example of which is a power-supply section.
Since the Multilayered planar inductors essentially have an open magnetic circuit, it is difficult to achieve the following two requirements:
(1) They have no leakage fluxes, and only slightly influence the other components of the IC in which they are in corporated. PA1 (2) They have a large inductance.
Therefore, the multilayered planar inductors cannot serve to provide sufficiently small integrated LC-circuit sections, such as a power-supply section.
Hence, there is still great demand for planar magnetic elements for use in a circuit section, which only slightly influence the other components of the circuit, influence other components. Further, the conventional planar magnetic elements can hardly have trimming function, due to the structural restriction imposed on them.